A 3GPP Evolved Packet System (EPS) comprises an Evolved Universal Terrestrial Radio Access Network (E-UTRAN), a Mobility Management Entity (MME), a Serving Gateway (S-GW), a Packet Data Network Gateway (PDN GW), a Home Subscriber Server (HSS), a 3GPP Authorization Authentication Accounting Server (3GPP AAA Server), a Policy and Charging Rules Function (PCRF) Server and other support nodes.
Wherein, the MME is responsible for the control plane related operations, such as mobility management, the non-access layer signaling processing and management of user mobile management context and so on.
The S-GW is an access gateway equipment which connects to E-UTRAN, forwards data between the E-UTRAN and the PDN GW, and is responsible for buffering paging waiting data.
The P-GW is the border gateway of the 3GPP Evolved Packet System (EPS) and the Packet Data Network (PDN), and responsible for functions such as PDN accessing and forwarding data between EPS and PDN.
The PCRF is a policy and charging rules function entity. It interacts with an Application Function (AF, such as the interface of an IP service provided by an operator) through a Rx interface to obtain service information. Meanwhile, the PCRF connects to the gateway equipments in the network through S7/S7a/S7c to be responsible for originating the IP bearer setup, assuring the QoS of the service data, and controlling charging.
The EPS supports for intercommunication with non-3GPP network. Intercommunication with non-3GPP network is implemented by an S2a/S2b/S2c interface. And the P-GW is used as the anchor between 3GPP and non-3GPP network.
The system architecture of EPS shows in FIG. 1. Wherein, non-3GPP access networks are classified into trusted non-3GPP IP access networks and untrusted non-3GPP IP access networks i.e. non-trusted non-3GPP IP access networks. The trusted non-3GPP IP access network can interface with the P-GW through the S2a; the untrusted non-3GPP IP access networks should connect to P-GW through an Evolved Packet Data Gateway (ePDG). The interface between ePDG and P-GW is an S2b. 
In traditional 3GPP networks, a Policy and Charging Enforcement Function (PCEF) only exists in the P-GW, and the PCRF can implement control of all the functions just by interfacing with the P-GW. The PCRF and the P-GW exchange information through a S7 interface (please see in FIG. 1).
However, after the 3GPP network provides the functions of intercommunicating with the non-3GPP network, if the interface between P-GW and S-GW bases on Proxy MIP (PMIP), the part of the policy enforcement function in the PCEF functions, which are called Gateway Control Function (GWCF), also exists in the S-GW, such as the bearer binding functions and so on. The S-GW and the PCRF exchange information by an S7c interface (please see in FIG. 1). When trusted non-3GPP access systems access a 3GPP network for intercommunicating, GWCF also resides in the trusted non-3GPP access gateway.
There is a plurality of PCRF nodes existing in one PLMS of the EPS. All the PCRF nodes belong to one or more Diameter (PCRF) domains (namely the PCRF Cloud in the figure), and all the PCRF in one Diameter (PCRF) domain are equivalent. One connection from a UE to a PDN network is called one IP Connectivity Access Network (IP-CAN) session. In order to ensure that the Policy and Charging Control (PCC) policies of one IP-CAN session are determined by one PCRF, the EPS introduces a logical function module of Diameter Routing Agent (DRA) into each Diameter (PCRF) domain. When a UE originates an IP-CAN session setup, the DRA selects a PCRF for this IP-CAN session.
However, in a non-roaming scenario, there is not a particular solution about how to associate all the PCEF, GWCF and AF related to one IP-CAN session with the same PCRF via DRA.